As an approach to a wide angle millimeter-wave radar, an article in the Proceedings of the 2001 IEICE General Conference (Engineering Sciences), (Japan), Article No. A-17-10, p. 391, discusses that the monopulse method is used to decrease the number of antenna elements intentionally to provide a wider angle detection range for the purpose of direction detection. The monopulse method employs a plurality of receiving antennas and determines the direction of an obstacle according to amplitude or phase difference between received signals (for example, refer to “Monopulse Principles and Techniques” authored by Samuel M. Sherman, (US), the first edition, Artech House, Feb. 1, 1984, pp. 8–19, or U.S. Pat. No. 6,243,052). Also, Japanese Patent Lid-open No. 2000-022423 discloses a mechanical scanning method in which a primary radiator of a dielectric lens antenna is moved with respect to the focal plane to scan beams.